Process For Producing A Glazed Ceramic Body

ABSTRACT

The invention also relates to the use of a glazing material for glazing a non-densely sintered substrate material.

The present invention relates to a process for glazing a ceramic bodythat has not yet been densely sintered.

Ceramic materials such as oxide ceramics are often used for theproduction of fully anatomical dental restorations. These offer highclinical safety, are usually metal-free, can also be used in minimallyinvasive preparations and are very attractive in terms of price incomparison with other metal-free restorations. However, a disadvantageis the numerous work steps which are usually required for the productionof such restorations.

The restorations are usually milled or ground out of presintered blanks,optionally characterized in terms of colour, densely sintered by thermaltreatment, further characterized by means of painting techniques andfinally glazed. As a rule, the glazing is carried out by applying aglaze to densely sintered restorations and thermal treatment in thetemperature range of from 700 to 950° C.

In addition, the application of veneering materials to the surface ofpartially sintered oxide ceramics is also known. However, in this case,the veneering materials infiltrate into the ceramic during the thermaltreatment.

Thus, in U.S. Pat. Nos. 4,626,392 A, 5,447,967 A and WO 99/52467 A1,methods are described in which a veneering material diffuses into anoxide ceramic and, with this, forms an inorganic-inorganic compositematerial at the surface between the polycrystalline substrate and theveneer.

In WO 2011/050786 A2 and US 2012/225201 A1 an adhesion promoter isdescribed which is applied to a partially sintered oxide ceramic anddiffuses into the surface during the sintering. After thedense-sintering step, a veneering ceramic is then applied and sinteredagain. The adhesion promoter represents the joining element between theoxide ceramic and the veneering ceramic sintered onto it.

In addition, so-called glass-infiltrated ceramics are known in dentaltechnology which are produced by infiltrating mostly silicate materialsinto porous oxide ceramic materials, wherein penetrated structures withaltered properties are usually formed. Thus, WO 2008/060451 A2 describesso-called sandwich structures which are produced by infiltratingglass-ceramic materials into substrates made from non-finally sinteredZrO₂. Particular properties can, in part, be positively influenced inthis way, but the optical properties in particular are often impaired.

WO 2005/070322 A1 and US 2005/164045 A1 describe a process in which asol is infiltrated into a substrate at room temperature under vacuum anddense sintering is then carried out.

The object underlying the invention is therefore to provide an improvedprocess for producing glazed ceramic bodies that avoids theabove-mentioned disadvantages and is characterized by a smaller numberof process steps, without impairing the optical and other physical andchemical properties of the ceramic.

This object is achieved by the process for producing a glazed ceramicbody according to claims 1 to 18. A subject of the invention is also theuse of a glazing material for glazing a substrate material according toclaim 19.

The process according to the invention for producing a glazed ceramicbody is characterized in that

-   -   (a) a glazing material is applied to a non-densely sintered        substrate material and    -   (b) the substrate material and the glazing material are        subjected to a heat treatment in a temperature range which        extends from a first temperature T₁ to a second temperature T₂,        which is higher than the first temperature, in order to obtain        the glazed body, wherein, at the temperature T₁, the glazing        material has a viscosity of more than 10^(2.5) Pa·s and, at the        temperature T₂, a viscosity of less than 10⁹ Pa·s.

At the temperature T₁, the glazing material preferably has a viscosityof more than 10^(4.0) Pa·s, in particular more than 10^(5.6) Pa·s andparticularly preferred more than 10^(7.0) Pa·s, and, at the temperatureT₂, a viscosity of preferably less than 10⁷ Pa·s and in particular lessthan 10^(5.6) Pa·s. It is particularly preferred that the glazingmaterial has, at the temperature T₁, has a viscosity of more than10^(5.6) Pa·s, in particular more than 10^(7.0) Pa·s, and, at thetemperature T₂, a viscosity of less than 10^(5.6) Pa·s.

The viscosity of the glazing material can, in particular, be determinedusing a viscosity-temperature curve based on the Vogel-Furcher-Tammannequation (VFT equation)

${\log_{10}(\eta)} = {A + \frac{B}{T - T_{0}}}$

-   -   η: dynamic viscosity at the temperature T    -   A, B, T₀: substance-specific constants.

This equation is solved starting from at least three and preferably fivepairs of values of characteristic temperatures determined experimentallyby means of a dilatometer or heating microscope, respectively, and theassociated viscosity values:

Designation and measurement method η (Pa · s) T_(g) (glass transitionpoint from dilatometer) 12 T_(d) (softening temperature fromdilatometer) 10 T_(S) (softening point from heating microscope) 5.6T_(HB) (hemisphere point from heating microscope) 3.5 T_(F) (flow pointfrom heating microscope) 2.1

The equation is solved by an approximation method according to theleast-squares method.

It has surprisingly been shown that the process according to theinvention allows direct glazing of not yet densely sintered substrateswithout the glazing material penetrating into the substrate material toa significant extent. The complex double heat treatment fordense-sintering and glazing can, in principle, thereby be dispensed withwithout a substantial change in the properties of the substrate materialoccurring through infiltration.

The non-densely sintered substrate material is, for example, anunsintered and preferably a presintered substrate material. Thenon-densely sintered substrate material usually has a relative densityin the range of from 30 to 90%, in particular in the range of from 40 to80% and preferably in the range of from 50 to 70%, based on the truedensity of the substrate material. It is preferred that the substratematerial starts to sinter at the temperature T₁. It is further preferredthat the substrate material is densely sintered at the temperature T₂.The substrate material is preferably kept at the temperature T₂ for aduration of 5 to 120 minutes, in particular 10 to 60 minutes and evenmore preferred 20 to 30 minutes. At the temperature T₂, the substratematerial typically has a relative density of at least 97%, in particularat least 98%, preferably at least 99% and most preferably at least99.5%, based on the true density of the substrate material.

The relative density is the ratio of the density of the substratematerial to the true density of the substrate material.

The density of the substrate material can be determined by weighing itand geometrically determining its volume. The density is then calculatedin accordance with the known formula

density=mass/volume.

The true density of the substrate material is determined by grinding thesubstrate material to a powder with an average particle size of 10 to 30μm, in particular of 20 μm, based on the number of particles, anddetermining the density of the powder by means of a pycnometer. Thedetermination of the particle size can be carried out, for example, withthe CILAS° Particle Size Analyzer 1064 from Quantachrome GmbH & Co. KGusing laser diffraction in accordance with ISO 13320 (2009).

In a preferred embodiment, at a temperature T_(x), at which thesubstrate material has a relative density of 95% based on the truedensity of the substrate material, the glazing material has a viscosityof more than 10².⁵ Pa s and preferably more than 10^(4.0) Pa·s. In aparticularly preferred embodiment, the glazing material has, at thetemperature T₁, a viscosity of more than 10^(5.6) Pa·s and in particularmore than 10^(7.0) Pa·s, at the temperature T_(x), at which thesubstrate material has a relative density of 95%, based on the truedensity of the substrate material, a viscosity of more than 10^(2.5)Pa·s and preferably more than 10^(4.0) Pa·s and, at the temperature T₂,a viscosity of less than 10⁹ Pa·s, in particular less than 10⁷ Pa·s andpreferably less than 10^(5.6) Pa·s. In an even further preferredembodiment, the glazing material has, at the temperature T₁, a viscosityof more than 10^(7.0) Pa·s, at the temperature T_(x), at which thesubstrate material has a relative density of 95%, based on the truedensity of the substrate material, a viscosity of more than 10^(4.0)Pa·s and, at the temperature T₂, a viscosity of less than 10^(5.6) Pa·s.

The process according to the invention is suitable for a wide variety ofceramic substrate materials. Examples of suitable substrate materialsare oxide ceramics, in particular oxide ceramics based on ZrO₂, Al₂O₃ orinorganic-inorganic composite materials, as well as glasses and glassceramics. Particularly preferred substrate materials are oxide ceramicsbased on ZrO₂ and in particular based on nanoscale ZrO₂.

In a particularly preferred embodiment of the invention the substratematerial comprises at least two layers, which differ in their chemicalcomposition and/or in particular in their colour.

As a rule, at a temperature of 950° C., the glazing material used in theprocess according to the invention has a viscosity of more than 10^(2.5)Pa·s. Furthermore, at a temperature of 1300° C., it typically has aviscosity of more than 10^(2.5) Pa·s. In addition, as a rule, at atemperature of 1450° C., it has a viscosity of less than 10⁹ Pa·s.Preferred glazing materials are characterized in that, at a temperatureof 950° C., they have a viscosity of more than 10^(4.0) Pa·s, preferablymore than 10^(5.6) Pa·s and particularly preferably more than 10^(7.0)Pa∜s, at a temperature of 1300° C., have a viscosity of more than 10⁴Pa·s and/or, at a temperature of 1450° C., have a viscosity of less than10⁷ Pa·s and preferably less than 10^(5.6) Pa·s.

Furthermore, glazing materials are preferred according to the inventionwhich, at a temperature of 700° C., have a viscosity of more than10^(2.5) Pa·s, at a temperature of 900° C., a viscosity of more than10^(2.5) Pa·s and/or, at a temperature of 1100° C., a viscosity of lessthan 10⁹ Pa·s. Particularly preferred glazing materials arecharacterized in that, at a temperature of 700° C., they have aviscosity of more than 10^(4.0) Pa·s, preferably more than 10^(5.6) Pa·sand particularly preferably more than 10^(7.0) Pa·s, at a temperature of900° C., have a viscosity of more than 10⁴ Pa·s and/or, at a temperatureof 1100° C., have a viscosity of less than 10⁷ Pa·s and preferably lessthan 10^(5.6) Pa·s.

The glazing material preferably comprises a frit. Particularly suitableare glazing materials which contain SiO₂, Al₂O₃ and K₂O and/or Na₂O.Glazing materials are preferred which contain at least one andpreferably all of the following components in the given amounts:

Component wt.-% SiO₂ 50.0 to 80.0, preferably 60.0 to 70.0 Al₂O₃ 10.0 to30.0, preferably 15.0 to 25.0 K₂O 0 to 20.0, preferably 5.0 to 15.0 Na₂O0 to 10.0, preferably 0.5 to 5.0 CaO 0 to 10.0, preferably 0.1 to 1.0BaO 0 to 10.0, preferably 0.1 to 1.0.

According to the invention, the glazing material can be applied to thesubstrate material for example in the form of a powder, a slip, a sprayor a lacquer, by means of an airbrushing method or by means of atransfer material. In an embodiment, the glazing material is mixed witha carrier, in particular water, to form a slip and this slip is appliedto the substrate material for example with the aid of a brush. In apreferred embodiment, the glazing material is usually mixed in apressurized container with a suitable carrier, in particular apropellant, and applied to the substrate material in the form of aspray. In a further embodiment, the glazing material is mixed with aliquid carrier, in particular water, and applied to the substratematerial by means of an airbrushing method. Alternatively, the glazingmaterial can be applied in dry form to a substrate material wetted witha liquid, in particular water, by means of an airbrushing method. In yetanother embodiment, the glazing material is applied to a transfermaterial, in particular a film or an adhesive strip, and transferred tothe substrate material with the transfer material. The transfer materialcan subsequently be detached or removed by means of a thermal treatment.

In a preferred embodiment, the glazing material is used in the form of acomposition which further contains at least one carrier and/or solventand preferably at least one inorganic and/or organic filler.

The process according to the invention is characterized, above all, inthat the glazing material does not substantially penetrate into thesubstrate material.

The process according to the invention is suitable in particular for theproduction of glazed dental bodies. It is therefore preferred accordingto the invention that the glazed ceramic body is a glazed dental bodyand in particular a glazed dental restoration. It is furthermorepreferred that the non-densely sintered substrate material has the shapeof a dental restoration. Particularly preferred dental restorations arebridges, inlays, onlays, crowns, veneers and abutments.

The invention further relates to the use of a glazing material forglazing a non-densely sintered substrate material, in which the glazingmaterial is applied to the non-densely sintered substrate material andthe substrate material and the glazing material are subjected to a heattreatment in a temperature range which extends from a first temperatureT₁ to a second temperature T₂, which is higher than the firsttemperature. At the temperature T₁, the glazing material preferably hasa viscosity of more than 10^(2.5) Pa·s and, at the temperature T₂, aviscosity of preferably less than 10⁹ Pa·s. Preferred glazing materialsare characterized in that, at the temperature T₁, they have a viscosityof more than 10⁴.° Pa·s, preferably more than 10^(5.6) Pa·s andparticularly preferably more than 10^(7.0) Pa·s and/or, at thetemperature T₂, have a viscosity of less than 10⁷ Pa·s and preferablyless than 10^(5.6) Pa·s. Further preferred embodiments of the useaccording to the invention result from the above description of theprocess according to the invention.

The invention is explained in more detail in the following withreference to examples.

EXAMPLES Examples 1A-B

Preparation of the Glazing Materials

Two different glasses with the compositions given in Table I wereprepared as glazing materials according to the invention.

TABLE I Example 1A 1B Component wt.-% wt.-% SiO₂ 65.41 73.39 K₂O 11.6210.25 Na₂O 2.25 1.98 Al₂O₃ 20.72 14.38 Total 100.00 100.00

For this purpose, first of all 200 g of the raw materials quartz powder(SiO₂), potassium carbonate (K₂CO₃), sodium carbonate (Na₂CO₃) andaluminium oxyhydroxyhydrate (AlO (OH)×H₂O) were thoroughly mixed for 30min by means of a Turbula mixer.

From the homogeneous mixtures, cylindrical compacts with a diameter ofabout 40 mm and weighing about 25 g were produced uniaxially by means ofa hydraulic press at a pressure of 3 MPa. These compacts were heated upto 1000° C. in a furnace on a quartz dish over 6 h and held at thistemperature for a further 6 h. After cooling in the furnace, thecompacts were comminuted by means of a jaw crusher to a size of about1.5 mm and subsequently ground in a mortar grinder (Retsch R200) for 15min.

The calcined mixture was pressed by means of a hydraulic press at apressure of 3 MPa to form compacts with a diameter of about 40 mm. Thesecompacts were heated up to 1150° C. in a furnace on a quartz dish over 6h and held at this temperature for a further 6 h. After cooling in thefurnace, the compacts were comminuted by means of a jaw crusher to asize of about 1.5 mm and subsequently ground in a mortar grinder (RetschR200) for 15 min.

From the calcined mixture, tempered cakes weighing about 40 g wereproduced by means of a hand press and placed in the hot furnace(Nabertherm HT16/17) on quartz dishes with a little quartz powder asseparating agent at about 1000° C. The tempered cakes were then heatedat 10 K/min to 1450° C. (Example LA) and 1400° C. (Example 1B),respectively and held at this temperature for 1.5 h. After the holdingtime had finished, the tempered cakes were cooled for about 2 min in airand then quenched in a water bath. The blanks obtained in this way werecomminuted by means of a jaw crusher to a size of about 1.5 mm andsubsequently ground in a mortar grinder (Retsch R200) for 15 min.

Examples 2A-B

Determination of the Viscosity Properties of the Glazing Materials

For the determination of the viscosity properties of the glazingmaterials obtained in Examples 1A-B as a function of the temperature, aviscosity-temperature curve was calculated on the basis of theVogel-Furcher-Tammann equation (VFT equation):

${\log_{10}(\eta)} = {A + \frac{B}{T - T_{0}}}$

-   -   η: dynamic viscosity at the temperature T    -   A, B, T₀: substance-specific constants

For this purpose, at least three of the following characteristictemperatures were determined experimentally by means of a dilatometer ora heating microscope, respectively:

Designation and measurement method η (Pa · s) T_(g) (glass transitionpoint from dilatometer) 12 T_(d) (softening temperature fromdilatometer) 10 T_(S) (softening point from heating microscope) 5.6T_(HB) (hemisphere point from heating microscope) 3.5 T_(F) (flow pointfrom heating microscope) 2.1

The temperatures T_(g) and T_(d) were determined by means of adilatometer (Bahr-Thermoanalyse GmbH) with a quartz glass push-rod andholder. The material to be examined was heated at a heating rate of 5K/min to the softening point (maximum 1000° C.). During the measurement,the set-up was flushed with nitrogen.

The temperatures T_(S), T_(HB) and T_(F) were determined by means of aheating microscope (Hesse Instruments with EMA I software). The materialto be examined was heated in a tube furnace at a heating rate of 10K/min. The software automatically determines the characteristic changesin shape of the sample and assigns the corresponding temperature tothem.

Starting from the pairs of values of the experimentally determinedcharacteristic temperatures and the associated viscosity values statedabove, the VFT equation was solved by an approximation method accordingto the least-squares method with the aid of the solver function of theMicrosoft Excel 2016 MSO (16.0.4456.1003) 32-bit software.

The viscosity-temperature curves determined in this way for the glazingmaterials obtained in Examples 1A-B are shown in FIG. 1. For comparison,this also shows a viscosity-temperature curve determined analogously fora commercially available glazing material (IPS e.max CAD Crystall./GlazeSpray, Ivoclar Vivadent AG).

Examples 3A-B

Use of the glazing materials for glazing

Small plates (19 mm×15.4 mm×1.5 mm) were cut from commercially availableblanks made of presintered ZrO₂ (IPS e.max ZirCAD MO 0, Ivoclar VivadentAG) and these were used without further thermal pretreatment assubstrate material. By means of an airbrushing method using a spray gun(VITA SPRAY-ON, Vita Zahnfabrik) at a working pressure of about 1 barand from a distance of about 10 cm, aqueous suspensions of the glazingmaterials A and B prepared in Examples 1A-B were sprayed onto thesesmall plates and, after drying in air for 30 min in a furnace (SintramatS1 1600, Ivoclar Vivadent AG, program P1), densely sintered within 70min and simultaneously glazed.

After the sintering, the glazed small ZrO₂ plates were embedded in atwo-component resin (EpoKwick Epoxy Resin/EpoKwick Epoxy Hardener 10:2,Buehler), polished at the boundary surface to optical quality (ApexDiamond Grinding Discs, Buehler, grain size to 0.5 μm) and subsequentlyexamined by means of scanning electron microscopy (SEM, backscatteredelectrons). The results are shown in FIG. 2A (Example 3A) and FIG. 2B(Example 3B). The results show that the glazing materials according tothe invention have not infiltrated into the substrate material inappreciable amounts.

Example 3C (Comparison)

Use of a Commercial Glazing Material for Glazing

Analogously to Examples 3A-B, small plates were cut from commerciallyavailable blanks made of presintered ZrO₂ (IPS e.max ZirCAD MO 0,Ivoclar Vivadent AG). By means of an airbrushing method using a spraygun (VITA SPRAY-ON, Vita Zahnfabrik) at a working pressure of about 1bar and from a distance of about 10 cm, an aqueous suspension of acommercially available glazing material (IPS e.max CAD Crystall./GlazeSpray, Ivoclar Vivadent AG) was sprayed onto these small plates and,after drying in air for 30 min in a furnace (Sintramat S1 1600, IvoclarVivadent AG, program P1), densely sintered within 70 min andsimultaneously glazed.

After the sintering, the glazed small ZrO₂ plates were embedded in atwo-component resin (EpoKwick Epoxy Resin/EpoKwick Epoxy Hardener 10:2,Buehler), polished at the boundary surface to optical quality (ApexDiamond Grinding Discs, Buehler, grain size to 0.5 μm) and subsequentlyexamined by means of scanning electron microscopy (SEM, backscatteredelectrons). The results are shown in FIG. 2C. These results show thatthe commercial glazing material has infiltrated into the substratematerial to a considerable extent.

Example 4

Influence of the Relative Density on the Infiltration Depth

Small plates (19 mm×15.4 mm×1.5 mm) were cut from commercially availableblanks made of presintered Zr0₂ (IPS e.max ZirCAD MO 0, Ivoclar VivadentAG) and these were presintered by thermal treatment to a relativedensity of 50%, 85%, 90% and 99.7%, respectively, in each case based onthe true density of the substrate material. By means of an airbrushingmethod using a spray gun (VITA SPRAY-ON, Vita Zahnfabrik) at a workingpressure of about 1 bar and from a distance of about 10 cm, an aqueoussuspension of the glazing material A prepared in Example LA was sprayedonto these small plates and, after drying in air for 30 min in a furnace(Sintramat S1 1600, Ivoclar Vivadent AG, program P7), densely sinteredwithin 70 min and simultaneously glazed.

After the sintering, the glazed small ZrO₂ plates were embedded in atwo-component resin (EpoKwick Epoxy Resin/EpoKwick Epoxy Hardener 10:2,Buehler), polished at the boundary surface to optical quality (ApexDiamond Grinding Discs, Buehler, grain size to 0.5 μm) and subsequentlyexamined by means of scanning electron microscopy (SEM, backscatteredelectrons) and energy dispersive x-ray spectroscopy (EDX). The resultsare shown in FIGS. 3A-D. From these it can be seen that the infiltrationdepth of the glazing material according to the invention is largelyindependent of the relative density and thus of the residual porosity ofthe substrate material.

1. Process for producing a glazed ceramic body, comprising (a) a glazingmaterial is applied to a non-densely sintered substrate material and (b)the substrate material and the glazing material are subjected to a heattreatment in a temperature range which extends from a first temperatureT_(I) to a second temperature T₂, which is higher than the firsttemperature, in order to obtain the glazed body, wherein, at thetemperature T₁, the glazing material has a viscosity of more than10^(2.5) Pa·s, and, at the temperature T₂, a viscosity of less than 10⁹Pa·s.
 2. Process according to claim 1, in which, at the temperature T₁,the glazing material has a viscosity of more than 10^(5.6) Pa·s, and, atthe temperature T₂, a viscosity of less than 10^(5.6) Pa·s.
 3. Processaccording to claim 1, in which the non-densely sintered substratematerial is a presintered substrate material.
 4. Process according toclaim 1, in which the non-densely sintered substrate material has arelative density in the range of from 30 to 90%, based on the truedensity of the substrate material.
 5. Process according to claim 1, inwhich the substrate material begins to sinter at the temperature T₁. 6.Process according to claim 1, in which the substrate material is kept atthe temperature T₂ for a period of 5 to 120 minutes.
 7. Processaccording to claim 1, in which, at the temperature T₂, the substratematerial has a relative density of at least 97%, based on the truedensity of the substrate material.
 8. Process according to claim 1, inwhich, at a temperature T_(x), at which the substrate material has arelative density of 95%, based on the true density of the substratematerial, the glazing material has a viscosity of more than 10^(2.5)Pa·s.
 9. Process according to claim 1, in which the glazing materialhas, at the temperature T₁, a viscosity of more than 10^(5.6) Pa·s, atthe temperature T_(x), at which the substrate material has a relativedensity of 95%, based on the true density of the substrate material, aviscosity of more than 10^(2.5) Pa·s, and, at the temperature T₂, aviscosity of less than 10⁹ Pa·s.
 10. Process according to claim 1, inwhich the substrate material is an oxide ceramic or aninorganic-inorganic composite material.
 11. Process according to claim1, in which the substrate material comprises at least two layers, whichdiffer in their colour.
 12. Process according to claim 1, in which, at atemperature of 950° C., the glazing material has a viscosity of morethan 10^(2.5) Pa·s, at a temperature of 1300° C., a viscosity of morethan 10^(2.5) Pa·s and, at a temperature of 1450° C., a viscosity ofless than 10⁹ Pa·s.
 13. Process according to claim 1, in which, at atemperature of 700° C., the glazing material has a viscosity of morethan 10^(2.5) Pa·s, at a temperature of 900° C., a viscosity of morethan 10^(2.5) Pa·s and, at a temperature of 1100° C., a viscosity ofless than 10⁹ Pa·s.
 14. Process according to claim 1, in which theglazing material comprises a frit, which comprises SiO₂, Al₂O₃ and K₂Oand/or Na₂O, and contains at least one or all of the followingcomponents in the given amounts: Component wt.-% SiO₂ 50.0 to 80.0 Al₂O₃10.0 to 30.0 K₂O 0 to 20.0 Na₂O 0 to 10.0 CaO 0 to 10.0 BaO 0 to 10.0


15. Process according to claim 1, in which the glazing material isapplied to the substrate material in the form of a powder, a slip, aspray or a lacquer, by an airbrushing method or incorporated into a filmor an adhesive strip.
 16. Process according to claim 1, in which theglazing material is used in the form of a composition which furthercomprises at least one carrier and/or solvent and/or at least oneinorganic and/or organic filler.
 17. Process according to claim 1, inwhich the glazing material does not substantially penetrate into thesubstrate material.
 18. Process according to claim 1, in which theglazed ceramic body is a glazed dental body or a glazed dentalrestoration or a bridge, an inlay, an onlay, a crown, a veneer, a facetor an abutment.
 19. Process of using a glazing material for glazing anon-densely sintered substrate material, wherein the glazing material isapplied to the non-densely sintered substrate material and the substratematerial and the glazing material are subjected to a heat treatment in atemperature range which extends from a first temperature T₁ to a secondtemperature T₂, which is higher than the first temperature, wherein, atthe temperature T₁, the glazing material has a viscosity of more than10^(2.5) Pa·s and, at the temperature T₂, a viscosity of less than 10⁹Pa·s.